The invention is directed generally to safety equipment which electrically senses an unsafe condition and produces an appropriate mechanical response. In particular, the invention is directed to an electromechanical undervoltage trip device which senses an undervoltage condition and causes the circuit breaker to trip.
Reliability is an essential characteristic of safety equipment. A trip device is a type of safety equipment which is useful to activate or trip a circuit breaker under appropriate conditions. Circuit breakers (hereinafter sometimes referred to as a breaker or breakers) are designed to open electrical circuits when activated. In some applications, the opening of a breaker initiates a chain of events which brings connected equipment to a safe state or stable condition. The sensitivity or the amount of tripping force necessary to trip a particular breaker bears directly on the characteristics of the particular trip devices used.
Some circuit breakers are tripped by the displacement of a trip bar. Trip devices capable of tripping such breakers, in addition to providing sufficient tripping force, must provide the necessary displacement to the trip bar to cause the breaker to trip. In some breakers, for example, motion of the trip bar releases a latch or other mechanism which otherwise holds the breaker contacts engaged. Springs or other force producing means often restrain or secure the latch against release. The force needed to release the latch depends in part on the force needed to move the latch against the bias of the spring. Frictional and inertial forces also affect the force needed to release the latch. Large safety breakers often require significant tripping force and significant trip bar displacement to cause a trip. Thus, in certain applications, forceful and significant motion must be provided in order to effect a breaker trip.
The term undervoltage as used herein encompasses a variety of electrical conditions. For example, in addition to voltage, which is the parameter specifically discussed herein, other electrical parameters (e.g., current, power, and so on) may be monitored by the apparatus of the present invention. It is intended that the monitoring of such other parameters falls within the scope of the invention.
Undervoltage trip devices operate to produce a response when a monitored voltage drops below a given level that is indicative of an unsafe condition. Such response may include moving a lever or trip arm with a force and displacement sufficient to trip a breaker of a given configuration. Present undervoltage trip devices generally consist of complicated multi-stage mechanisms, including at least one sensor mechanism alone and at least one actuation mechanism. Multi-stage mechanisms are often required because the response of one sensor, or one actuation mechanism alone, creates insufficient force or displacement to directly trip a circuit breaker.
In some devices, the sensor mechanism consists of an electromagnetic coil, which, when energized, produces a force sufficient to restrain a mass against an opposing force, such as the bias of a spring or gravity. When the voltage applied to the coil falls below a predetermined value, the spring bias or gravity overcomes the opposing force produced by the Coil and thereby allows the mass to move.
The actuating mechanism in such a device often includes a combination of friction latches, levers and springs which cooperate to sense the motion of the mass and to produce a mechanical response having a force and displacement sufficient to trip the circuit breaker. The actuation mechanism itself may consist of multiple stages designed to produce force and displacement amplification, with each state producing a more forceful and more perceptible displacement output than the preceding stage.
Prior undervoltage trip devices are susceptible to failure in a number of failure modes. In one mode, sometimes referred to as the frictional failure mode, the frictional forces associated with sensing mechanisms increase over time to a point where the force required to move the mass becomes too great, or the force produced by the moving mass becomes insufficient to activate the actuation mechanisms. Likewise, actuation mechanisms may experience increased frictional forces over time, whereby they become disabled or inoperative. In another failure mode, sometimes called the output failure mode, the force produced by the trip device decreases to a point where the device can no longer trip the breaker, even if all the various mechanisms operate.
Multi-stage devices are complex, often requiring many precision parts which are difficult to assemble. In order to reduce the possibility of failure, such devices require strict manufacturing tolerances for the many parts, extensive inspection of parts and sub-assemblies, rigorous testing, and frequent and complex maintenance schedules (e.g., lubrication) after installation. The aforementioned characteristics of multi-stage devices and their particular manufacturing requirements result in a high cost of manufacture and significant maintenance costs to ensure reliability.